Retroreflective articles having a machine-readable code

ABSTRACT

Retroreflective articles comprise a substrate and a bar code provided on the substrate. The bar code comprises at least one human-readable information which provides framing information and a machine-readable information which provides variable information. The human-readable information is visible under a first condition and invisible under a second condition, and the machine-readable information is invisible under the first condition and visible under the second condition.

TECHNICAL FIELD

The present application relates generally to retroreflective articleshaving a machine-readable code that are useful, for example, in machinevision systems.

BACKGROUND

Optimally, license plates have an overall similarity of styling orappearance that enables rapid recognition of license plates issued byvarious licensing authorities and that inhibits counterfeiting. At thesame time, license plates should provide a distinct, individualized andunique identifying code or image for each vehicle, state, or driver. Tothat end, many licensing authorities offer “vanity” license plates. Suchplates allow the driver to select an attractive or meaningful design ormessage that will be printed on their license plate. The production ofsuch “vanity” license plates results in each state offering numerousdifferent license plate designs to its constituents.

Meanwhile, automated enforcement systems, including, for example,electronic toll systems, red light miming systems, speed enforcementsystems, and access control systems, are becoming more prevalent. Manyembodiments of such systems rely on an accurate reading of a vehicle'slicense plate, which is often performed by an automated license platerecognition (ALPR) system. However, obtaining an accurate reading of avehicle's license plate is becoming increasingly difficult due to thewide variety of license plates now on the roads.

Automatic Vehicle Identification (AVI) is a term applied to thedetection and recognition of a vehicle by an electronic system.Exemplary uses for AVI include, for example, automatic tolling, trafficlaw enforcement, searching for vehicles associated with crimes, andfacility access control. Ideal AVI systems are universal (i.e., they areable to uniquely identify all vehicles with 100% accuracy). The two maintypes of AVI systems in use today are (1) systems using RFID technologyto read an RFID tag attached to a vehicle and (2) systems using a cameraor optical device and a computer to read a machine-readable codeattached to a vehicle.

One advantage of RFID systems is their high accuracy, which is achievedby virtue of error detection and correction information contained on theRFID tag. Using well known mathematical techniques (cyclic redundancycheck, or CRC, for example), the probability that a read is accurate (orthe inverse) can be determined. However, RFID systems have somedisadvantages, including that not all vehicles include RFID tags. Also,existing unpowered “passive” RFID tag readers may have difficultypinpointing the exact location of an object. Rather, they simply reportthe presence or absence of a tag in their field of sensitivity.Moreover, many RFID tag readers only operate at short range, functionpoorly in the presence of metal, and are blocked by interference whenmany tagged objects are present. Some of these problems can be overcomeby using active RFID technology or similar methods. However, thesetechniques require expensive, power-consuming electronics and batteries,and they still may not determine position accurately when attached todense or metallic objects.

Machine vision systems (often called Automated License Plate Readers orALPR systems) use a machine or device to read a machine-readable codeattached to a vehicle. In many embodiments, the machine-readable code isattached to, printed on, or adjacent to a license plate. One advantageof ALPR systems is that they are can be used almost universally, sincealmost all areas of the world require that vehicles have license plateswith visually identifiable information thereon. However, the task ofrecognizing visual tags can be complicated. For example, the readaccuracy from an ALPR system is largely dependent on the quality of thecaptured image as assessed by the reader. Existing systems havedifficulty distinguishing tags from complex backgrounds and handlingvariable lighting. Further, the accuracy of ALPR systems suffers whenlicense plates are obscured or dirty.

Some exemplary ALPR systems include a bar code (or othermachine-readable portion) containing an identification code which willprovide information about the vehicle. Typically, the bar code on alicense plate includes inventory control information (i.e., a small barcode not intended to be read by the ALPR). Some publications (e.g.,European Patent Publication No. 0416742 and U.S. Pat. No. 6,832,728)discuss including one or more of owner information, serial numbers,vehicle type, vehicle weight, plate number, state, plate type, andcounty on a machine-readable portion of a license plate.

SUMMARY

In one aspect, the inventors of the present application sought todevelop a bar code wherein framing and variable information are obtainedunder two different conditions. In some embodiments, the framinginformation is provided by human-readable information, and variableinformation is provided by machine-readable information.

In another aspect, the inventors of the present application sought tomake automated license plate identification easier while maintaining theaesthetics of the license plate design. License plates can bechallenging for an automated license plate recognition system to readdue to at least some of the following factors: (1) varying reflectiveproperties of the license plate materials; (2) non-standard fonts,characters, and designs on the license plates; (3) varying embeddedsecurity technologies in the license plates; (4) variations in thecameras or optical character recognition systems; (5) the speed of thevehicle passing the camera or optical character recognition system; (6)the volume of vehicles flowing past the cameras or optical characterrecognition systems; (7) the spacing of vehicles flowing past thecameras or optical character recognition systems; (8) wide variances inambient illumination surrounding the license plates; (9) weather; (10)license plate mounting location and/or tilt; (11) wide variances inlicense plate graphics; (12) the detector-to-license plate-distancepermissible for each automated enforcement system; and (13) occlusion ofthe license plate by, for example, other vehicles, dirt on the licenseplate, articles on the roadway, natural barriers, etc. The inventors ofthe present application sought to improve the accuracy of automatedlicense plate identification without significantly changing theappearance of the license plate under a first condition. In someembodiments, a license plate comprising an identification system (i.e.,a machine-readable bar code) which is invisible to a viewer under afirst condition, such as, for example, ambient visible condition, isprovided.

One embodiment of the present application relates to a retroreflectivearticle comprising a retroreflective substrate having at least onehuman-readable information and an embedded machine-readable information,wherein the embedded machine-readable information is contained withinthe boundaries of at least a portion of the human-readable information.

Another embodiment of the present application relates to an articlecomprising a substrate and a bar code provided on the substrate, the barcode comprising at least one human-readable information which providesframing information and a machine-readable information which providesvariable information; wherein the human-readable information is visibleunder a first condition and invisible under a second condition and themachine-readable information is invisible under the first condition andvisible under the second condition.

Yet another embodiment of the present application relates to a bar codecomprising framing information and variable information, wherein theframing information is visible under a first condition and invisibleunder a second condition, and the variable information is invisibleunder the first condition and visible under the second condition.

As used herein, the term “machine-readable information” refers toinformation that is encoded in a form that can be optically imaged by amachine or computer and interpreted by its hardware and software.

As used herein, the term “human-readable information” refers toinformation that can be read and comprehended by a human.

As used herein, the term “contained” means that the spatial features ofembedded machine-readable information overlap with and are locatedwithin the spatial limits of at least a portion of human-readableinformation.

As used herein the terms “invisible” and “not visible” mean not easilydetectable and/or not visually noticeable by an optical detector. Incontrast, the term “visible”, as used herein, means easily detectableand/or visually noticeable by an optical detector.

As used herein, a “machine-readable bar code” requires framinginformation and variable information. In some embodiments of the presentapplication, a machine-readable bar code is provided wherein framinginformation is provided by human-readable information and variableinformation is provided by machine-readable information.

As used herein, “optically inactive” means not retroreflective.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exploded view of an exemplary motor vehicle license plate.

FIG. 2 is digital photograph of the license plate of FIG. 1 underambient visible conditions.

FIG. 3 is a digital photograph of the license plate of FIG. 1 undervisible retroreflective conditions.

FIG. 4 is a digital photograph of the license plate of FIG. 1 underretroreflective infrared conditions.

FIG. 5 is a digital photograph of the license plate of FIG. 1 underoff-axis infrared illumination conditions.

FIG. 6 is digital photograph of an exemplary license plate under ambientvisible conditions.

FIG. 7 is a digital photograph of the license plate of FIG. 6 undervisible retroreflective conditions.

FIG. 8 is a digital photograph of the license plate of FIG. 6 underretroreflective infrared conditions.

FIG. 9 is a digital photograph of the license plate of FIG. 6 underoff-axis infrared illumination conditions.

FIG. 10 is a digital photograph of an exemplary optically inactivearticle under visible ambient conditions.

FIG. 11 is a digital photograph of the optically inactive article ofFIG. 10 under off-axis infrared illumination conditions.

FIG. 12 is a digital photograph of an exemplary retroreflective articleunder visible ambient conditions.

FIG. 13 is a digital photograph of the retroreflective article of FIG.12 under visible retroreflective conditions.

DETAILED DESCRIPTION

Various embodiments and implementations will be described in detail.These embodiments should not be construed as limiting the scope of thepresent application in any manner, and changes and modifications may bemade without departing from the spirit and scope of the inventions. Forexample, many of the embodiments, implementations, and examples arediscussed with specific reference to license plates, but these shouldnot be construed to limit the application scope to this one exemplaryimplementation. Further, only some end uses have been discussed herein,but end uses not specifically described herein are included within thescope of the present application. As such, the scope of the presentapplication should be determined by the claims.

In a first embodiment, the present application relates to aretroreflective article comprising: a retroreflective substrate havingat least one human-readable information and an embedded machine-readableinformation, wherein the embedded machine-readable information iscontained within the boundaries of at least a portion of thehuman-readable information.

In a second embodiment, the present application relates to theretroreflective article of the first embodiment, wherein thehuman-readable information is visible under a first condition andinvisible under a second condition, and the embedded machine-readableinformation is invisible under the first condition and visible under thesecond condition.

In a third embodiment, the present application relates to theretroreflective article of the second embodiment, wherein the firstcondition is a first spectral range or first lighting condition, and thesecond condition is a second spectral range or a second lightingcondition.

In a fourth embodiment, the present application relates theretroreflective article the third embodiment, wherein the first spectralrange is between about 400 nm and about 700 nm and the second spectralrange is between about 700 nm and about 1100 nm.

In a fifth embodiment, the present application relates to theretroreflective article of the third embodiment, wherein the firstlighting condition is ambient visible condition and the second lightingcondition is visible retroreflective condition.

In a sixth embodiment, the present application relates to aretroreflective article as in any one of the preceding embodiments,wherein the at least one human-readable information and the embeddedmachine-readable information form a bar code.

In a seventh embodiment, the present application relates to theretroreflective article of the sixth embodiment, wherein thehuman-readable information provides framing information and the embeddedmachine-readable information provides variable information.

In an eighth embodiment, the present application relates to aretroreflective article as in any one of the preceding embodiments,wherein the retroreflective substrate includes prismatic sheeting orbeaded sheeting.

In a ninth embodiment, the present application relates a retroreflectivearticle as in any one of the preceding embodiments, wherein thehuman-readable information comprises at least a portion of at least onealphanumeric character or a geometric shape.

In a tenth embodiment, the present application relates to aretroreflective article as in any one of the preceding embodiments,wherein the article is a license plate.

In an eleventh embodiment, the present application relates to an articlecomprising a substrate and a bar code provided on the substrate, the barcode comprising at least one human-readable information which providesframing information and a machine-readable information which providesvariable information; wherein the human-readable information is visibleunder a first condition and invisible under a second condition, and themachine-readable information is invisible under the first condition andvisible under the second condition.

In a twelfth embodiment, the present application relates the article ofthe eleventh embodiment, wherein the substrate is retroreflective.

In a thirteenth embodiment, the present application relates to anarticle as in one of embodiments 11 and 12, wherein the first conditionis a first spectral range or a first lighting condition, and the secondcondition is a second spectral range or a second lighting condition.

In a fourteenth embodiment, the present application relates to thearticle of the thirteenth embodiment, wherein the first spectral rangeis between about 400 nm and about 700 nm and the second spectral rangeis between about 700 nm and about 1100 nm.

In a fifteenth embodiment, the present application relates to thearticle of the thirteenth embodiment, wherein the first lightingcondition is ambient visible condition and the second lighting conditionis visible retroreflective condition.

In a sixteenth embodiment, the present application relates to an articleas in one of embodiments 11-15, wherein the human-readable informationcomprises at least a portion of at least one alphanumeric character.

In a seventeenth embodiment, the present application relates to anarticle as in one of embodiments 11-16, wherein the machine-readableinformation is contained within the boundaries of at least a portion ofthe human-readable information.

In an eighteenth embodiment, the present application relates to anarticle as in one of embodiments 11-17, wherein the substrate isretroreflective.

In a nineteenth embodiment, the present application relates to thearticle of embodiment 18, wherein the retroreflective substrate includesprismatic sheeting or beaded sheeting.

In a twentieth embodiment, the present application relates to an articleas in one of embodiments 11-19, wherein the article is a license plate.

In a twenty-first embodiment, the present application relates to a barcode comprising framing information and variable information, whereinthe framing information is visible under a first condition and invisibleunder a second condition, and the variable information is invisibleunder the first condition and visible under the second condition.

In a twenty-second embodiment, the present application relates the barcode of embodiment 21, wherein the first condition is a first spectralrange or first lighting condition, and the second condition is a secondspectral range or a second lighting condition.

In a twenty-third embodiment, the present application relates the barcode of embodiment 22, wherein the first spectral range is between about400 nm and about 700 nm and the second spectral range is between about700 nm and about 1100 nm.

In a twenty-fourth embodiment, the present application relates the barcode of embodiment 22, wherein the first lighting condition is ambientvisible condition and the second lighting condition is visibleretroreflective condition.

In a twenty-fifth embodiment, the present application relates an articlecomprising a bar code as in one of embodiments 21-24.

In a twenty-sixth embodiment, the present application relates to thearticle of embodiment 25, wherein the article is a license plate.

In a twenty-seventh embodiment, the present application relates toautomated license plate reader systems comprising the article ofembodiment 10, 20 or 26 and a detector.

In a twenty-eighth embodiment, the present application relates to thesystem of embodiment 27, wherein the detector comprises a camera and alighting system.

In at least some preferred embodiments, the retroreflective article(e.g., license plate) has both human-readable information andmachine-readable information. Examples of human-readable informationinclude, but are not limited to, alphanumeric characters, designs,geometric shapes, symbols, and Asian language characters. As usedherein, the term “machine-readable information” refers to informationthat is encoded in a form that can be optically imaged by a machine orcomputer and interpreted by its hardware and software, but not by ahuman. Theoretically, anything that can be read by a human can also beread by a machine, although it may not necessarily be comprehended by ahuman. As used herein, the term “human-readable information” refers toinformation that can be read and comprehended by a human. Exemplarytypes of machine-readable information include, for example, bar codes,color bar codes, 2D bar codes, geometric symbols as described inEuropean Publication No. 0416742 and the like. The machine-readableinformation can be, for example, visible or invisible to a detector(e.g., human eye, camera).

Machine-readable bar codes require framing information and variableinformation. Framing information in a bar code is used to allow locationof the variable information within the bar code. Examples of framinginformation include 8 bit by 8 bit squares in each of three corners ofQR (QUICK RESPONSE) bar codes. Data Matrix 2D bar codes use a dark baralong two edges and alternating dark and light bits along the other twoedges as framing information. In the existing bar codes, framinginformation is provided adjacent the variable information, and isobtained from the same image (e.g., taken under the same condition) fromwhich the variable information (e.g., data) is obtained.

In some embodiments, variable information is in the form of a binaryoptical code. In binary optical codes all areas of the code are dividedinto a set number and geometry of known regions. All regions in an imageare then classified as either light or dark. Light regions or pixelsrepresent a value (e.g., 0 (zero)) and dark regions represent anothervalue (e.g., 1). Large contrast (i.e., difference in brightness) betweenlight and dark regions allow for easier interpretation of the binaryoptical code.

In one aspect of the present application, the inventors sought todevelop a bar code in which at least a portion of the framinginformation is obtained from an image taken under a different conditionthan the condition used to obtain the variable information. In someembodiments, resolution of the images used to obtain framing andvariable information are not substantially different. As used herein,“resolution of the images” means that each pixel covers substantiallythe same amount of area of an object. In some embodiments, theresolution of the images is within a factor of about 1, 2, 5, or 10. Insome embodiments, framing information is obtained from an image takenunder a first spectral range, and the variable information is obtainedfrom an image taken under a second spectral range, different from thefirst spectral range. In some preferred embodiments, framing informationis visible under the first spectral range and invisible under the secondspectral range, and the variable information is invisible under thefirst spectral range and visible under the second spectral range. Asused herein the terms “invisible” and “not visible” mean not easilydetectable and/or not visually noticeable by an optical detector. Incontrast, the term “visible”, as used herein, means easily detectableand/or visually noticeable by an optical detector.

In some embodiments, framing information is obtained from an image takenunder a first lighting condition, and the variable information isobtained from an image taken under a second lighting condition,different from the first lighting condition. In some preferredembodiments, framing information is visible under the first lightingcondition and invisible under the second lighting condition, and thevariable information is invisible under the first lighting condition andvisible under the second lighting condition.

In some preferred embodiments, the human-readable information and themachine-readable information form a bar code. In some preferredembodiments, the human-readable information provides framing informationwhile the machine-readable information provides variable information. Inthe bar code of the present application, framing information (e.g.,human-readable information) may occupy at least some of the same spaceas the variable information (e.g., machine-readable information) on agiven article. The two images needed to obtain both framing and variableinformation are images of the same article obtained under differentspectral ranges, lighting conditions and/or relative motion conditions.As a result, the machine-readable information is concealed by thehuman-readable information under a first condition. In some embodiments,the human-readable information is visible to an optical detector (e.g.,human eye, camera) under a first spectral range and invisible to thedetector under a second spectral range, whereas the embeddedmachine-readable information is visible to the detector under the secondspectral range but invisible under the first spectral range. In someembodiments, the first spectral range is from about 400 nm to about 700nm (i.e., visible light spectrum) and the second spectral range is fromabout 700 nm to about 1100 nm (i.e., near infrared spectrum). In someembodiments, the human-readable information is visible to the detectorunder a first lighting condition and invisible to the detector under asecond lighting condition, whereas the embedded machine-readableinformation is visible to the detector under the second lightingcondition but invisible under the first lighting condition. In someembodiments, the first lighting condition is an ambient visiblecondition (i.e., diffuse visible light) and the second lightingcondition is a visible retroreflective condition (i.e., coaxial visiblelight). In some embodiments, the position of the light source(s) isdifferent in the first and second lighting conditions.

In some embodiments of the present application, the machine-readableinformation is an embedded machine-readable information. The embeddedmachine-readable information is completely contained within theboundaries of at least a portion of the human-readable information, andcompletely concealed by the human-readable information. The term“contained” as used herein means that the spatial features of theembedded machine-readable information overlap with and are locatedwithin the spatial limits of at least a portion of the human-readableinformation.

In one aspect of the present application, the inventors sought todevelop articles having the bar code described herein. Exemplaryarticles include retroreflective and optically inactive (i.e., nonretroreflective) articles. Examples of retroreflective articles include,but are not limited to, license plates, signage, and validationstickers. Examples of optically inactive articles include, but are notlimited to, graphic designs.

FIGS. 1 and 2 are, respectively, an exploded perspective view of anexemplary motor vehicle license plate and a digital photograph of thelicense plate as viewed under visible ambient conditions (i.e., underdiffuse visible light conditions) by an optical detector such as, forexample, a human or a digital camera. License plate 10 includes arectangular license plate blank 20 positioned adjacent to a rectangularpiece of retroreflective sheeting 40. License plate blank 20 includestwo major surfaces 22 and 24 separated by a thickness that is bound byfour side surfaces 26, 28, 30, and 32. License plate blank 20 may beformed of any material having the desired rigidity, for example, metal,plastic, or wood. Retroreflective sheeting 40 includes two majorsurfaces 42 and 44 separated by a thickness that is bound by four sidesurfaces 46, 48, 50, and 52. Printed on retroreflective sheeting 40 are:a Minnesota graphic 60; “Explore Minnesota.com” text 62; “JAN” text 63;“10,000 Lakes” text 64; the alphanumeric characters “001 SPL” 66; and awooded lake background graphic 68. License plate 10 may optionallyinclude four holes 70 cut through retroreflective sheeting 40 andlicense plate blank 20 that facilitate affixation of license plate 10 toa motor vehicle by, for example, screws or rivets. This is only oneexemplary affixation system and those of skill in the art willappreciate that others can be used. License plate blank 20 andretroreflective sheeting may optionally be held together and centered inthe middle of frame 72.

FIG. 3 is a digital photograph of the license plate of FIGS. 1 and 2shown as viewed under visible retroreflective conditions (i.e., coaxialvisible light illumination). A bar code is provided on license plate 10,where the alphanumeric characters 66 provide the framing information andthe embedded machine-readable information provides variable information80. In FIGS. 2 and 3, variable information 80 is invisible to, forexample, the human eye.

FIG. 4 is a digital photograph of the license plate of FIGS. 1 and 2shown as viewed under retroreflective infrared conditions (i.e., coaxialinfrared illumination). In FIG. 4, variable information 80 is completelycontained within the boundaries of the alphanumeric characters 66.Variable information 80 was provided by selectively adhering pieces ofvisible and infrared-scattering pressure sensitive adhesive tape “ScotchMagic Tape”, commercially available from 3M Company, of St. Paul, Minn.,over the alphanumeric characters 66. Variable information 80 overlapsalphanumeric characters 66. Under both ambient visible light conditionsand visible retroreflective conditions, as shown, respectively, in FIGS.2 and 3, variable information 80 is not visible, while alphanumericcharacters 66 are visible. Therefore, framing information may beobtained from images taken either under ambient visible conditions orvisible retroreflective conditions. Variable information 80 may beobtained from images taken under retroreflective infrared conditions, asshown in FIG. 4, since variable information 80 is visible under theseconditions while alphanumeric characters 66 are not.

FIG. 5 is a digital photograph of the license plate of FIGS. 1 and 2shown as viewed under off-axis infrared illumination conditions (i.e.,wherein infrared illumination is not coincident with the optical axis ofthe camera lens). Neither the alphanumeric characters 66 nor thevariable information 80 is visible under these conditions.

FIG. 6 is a digital photograph of yet another exemplary license plate asviewed under visible ambient conditions by an optical detector. Licenseplate 100 includes a rectangular license plate blank 120 positionedadjacent to a rectangular piece of retroreflective (e.g.,retroreflective) sheeting 140. Retroreflective sheeting 140 includes twomajor surfaces 142 and 144 separated by a thickness that is bound byfour side surfaces 146, 148, 150, and 152. Printed on retroreflectivesheeting 140 are: a Minnesota graphic 160; “Explore Minnesota.com” text162; “JAN” text 163; “10,000 Lakes” text 164; the alphanumericcharacters “SAM 123” 166; and a wooded lake background graphic 168.License plate 100 may optionally include four holes 170 cut throughretroreflective sheeting 140 and license plate blank 120 that facilitateaffixation of license plate 100 to a motor vehicle by, for example,screws or rivets. A bar code is provided on license plate 100, whereinthe alphanumeric characters 166 provide framing information and anembedded machine-readable information provides variable information 180.Variable information 180 is invisible under ambient visible conditions.

FIG. 7 is a digital photograph of the license plate of FIG. 6 shown asviewed under visible retroreflective conditions. In FIGS. 6 and 7,variable information 180 is invisible by an optical detector.

FIG. 8 is a digital photograph of the license plate of FIGS. 6 and 7shown as viewed under retroreflective infrared conditions. FIG. 8 showsthe variable information 180, which is completely contained within theboundaries of the alphanumeric characters 166. The alphanumericcharacters 166 were printed using infrared transparent CMY inks.Variable information 180 was provided by printing partial alphanumericcharacters using a visibly opaque, infrared-absorbing black ink.Variable information 180 overlaps alphanumeric characters 166. Underboth ambient visible light conditions and visible retroreflectiveconditions, as shown, respectively, in FIGS. 6 and 7, variableinformation 180 is not visible, while alphanumeric characters 166 arevisible. Under retroreflective infrared conditions, as shown in FIG. 7,variable information 180 is visible while alphanumeric characters 166are not visible.

FIG. 9 is a digital photograph of the license plate of FIG. 6 shown asviewed under off-axis infrared illumination conditions. Under theseconditions variable information 180 is visible while alphanumericcharacters 166 are not. Framing information of the bar code provided onlicense plate 100 may be obtained from images taken under ambientvisible conditions or under visible retroreflective conditions. Variableinformation may be obtained from images taken under infraredretroreflective conditions or off-axis infrared illumination conditions.

FIG. 10 is a digital photograph of an optically inactive article 200shown as viewed under visible ambient conditions. Machine-readableinformation 280 is invisible in under this condition and concealed byalphanumeric characters “SAM 123” 266. FIG. 11 is a digital photographshown as viewed under off-axis infrared illumination. Only themachine-readable information 280 is visible under this condition.

In some embodiments, the human-readable information is a design orgeometric shape which has no encoded information, such as, for example,a star, a rectangle, a circle, and a square. FIG. 12 is a digitalphotograph of a retroreflective article 300 shown as viewed undervisible ambient conditions. Machine-readable information 380 isinvisible under this condition, and concealed by human-readableinformation which, in this embodiment, is a red rectangle 371. FIG. 13is a digital photograph shown as viewed under visible retroreflectiveconditions in which the background is saturated. Machine-readableinformation 380 is visible under this condition, while human-readablerectangle 371 is not.

The retroreflective article chosen for any specific implementation willdepend on the desired optical, structural, and durabilitycharacteristics. As such, desirable retroreflective articles andmaterials will vary based on the intended application. Retroreflectivearticles and materials include reflective and retroreflectivesubstrates. The term “retroreflective” as used herein refers to theattribute of reflecting an obliquely incident light ray in a directionantiparallel to its incident direction, or nearly so, such that itreturns to the light source or the immediate vicinity thereof. Two knowntypes of retroreflective sheeting are microsphere-based sheeting andcube corner sheeting (often referred to as prismatic sheeting).Microsphere-based sheeting, often referred to as “beaded” sheeting,employs a multitude of microspheres typically at least partiallyembedded in a binder layer and having associated specular or diffusereflecting materials (e.g., pigment particles, metal flakes, vaporcoats) to retroreflect incident light. Illustrative examples aredescribed in, for example, U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat.No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult). Cube cornerretroreflective sheeting, often referred to as “prismatic” sheeting,comprises a body portion typically having a substantially planar frontsurface and a structured rear surface comprising a plurality of cubecorner elements. Each cube corner element comprises three approximatelymutually perpendicular optical faces. Illustrative examples aredescribed in, for example, U.S. Pat. No. 1,591,572 (Stimson), U.S. Pat.No. 4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appledorn et al.),U.S. Pat. No. 5,138,488 (Szczech), and U.S. Pat. No. 5,557,836 (Smith etal.). A seal layer may be applied to the structured surface to keepcontaminants away from individual cube corners. Flexible cube cornersheetings, such as those described, for example, in U.S. Pat. No.5,450,235 (Smith et al.) can also be incorporated in embodiments orimplementations of the present application. Retroreflective sheeting foruse in connection with the present application can be, for example,either matte or glossy.

The retroreflective articles described herein are ordinarily configuredto include sheeting that can be applied to a given object or substrate.The articles are generally optically single-sided. That is, one side(designated the front side) is generally adapted to both receiveincident light from a source and emit reflected or retroreflected lighttoward a detector (such as the eye of an observer), and the other side(designated the rear side) is generally adapted for application to anobject such as by an adhesive layer. The front side faces the lightsource as well as the detector. The articles do not generally transmitsignificant amounts of light from the front side to the rear side, orvice versa, due at least in part to the presence of a substance or layeron the retroreflector such as a vapor coat of metal, a seal film, and/oran adhesion layer.

One use for the retroreflective articles described herein is in licenseplates that are detected by a license plate detection or recognitionsystem. One exemplary license plate detection system uses a camera and alighting system to capture license plate images. An image of the sceneincluding the license plate can be made from, for example, ambientvisible conditions and from light added by a designated light source(for example, coaxial lighting that directs light rays onto the licenseplate when the camera is preparing to record an image). The light raysemitted by the coaxial lighting in combination with the retroreflectiveproperties of the license plate create a strong, bright signal from thelocation of the plate in the otherwise large image scene. The brightsignal is used to identify the location of the license plate. Then, theautomatic license plate recognition (ALPR) focuses on the region ofinterest (the region of brightness) and searches for matches to expectedhuman-readable information or embedded machine-readable information bylooking for recognizable patterns of contrast. In some embodiments, therecognized human-readable information provides framing information for abar code, while the embedded machine-readable information providesvariable information. Human-readable information is obtained under afirst spectral range, and embedded machine-readable information isobtained under a second spectral range, different from the firstspectral range.

The light in the driving and ALPR environment can be divided into thefollowing spectral regions: visible light in the region between about400 and about 700 nm and infrared light in the region between about 700and about 1100 nm. Typical cameras have sensitivity that includes bothof these ranges, although the sensitivity of a standard camera systemdecreases significantly for wavelengths longer than 1100 nm. Variouslight emitting diodes (LEDs) can emit light over this entire wavelengthrange, and typically most LEDs are characterized by a central wavelengthand a narrow distribution around that wavelength. For example, in asystem including LEDs emitting light rays having a wavelength of 830nm+/−20 nm, a suitably equipped camera could detect a license plate inthe near infrared spectrum with light not visible to the driver of thevehicle. Thus the driver would not see the “strobe” light effect of theLEDs and would not be distracted by them.

The cameras and lights for these systems are typically mounted to viewthe license plates at some angle to the direction of vehicle motion.Exemplary mounting locations include positions above the traffic flow orfrom the side of the roadway. Images are typically collected at an angleof 20 degrees to 45 degrees from normal incidence (head-on) to thelicense plate.

A detector which is sensitive to infrared or ultraviolet light asappropriate would be used to detect retroreflected light outside of thevisible spectrum. Exemplary cameras include those sold by Federal SignalTechnologies of Irvine Calif., including but not limited to the P372.

The retroreflective articles described herein can be used to improve thecapture efficiency of these license plate detection or recognitionsystems. ALPR capture can be described as the process of correctlylocating and identifying license plate data, including, but not limitedto, indicia, plate type, and plate origin. Applications for theseautomated systems include, but are not limited to, electronic tollsystems, red light running systems, speed enforcement systems, vehicletracking systems, trip timing systems, automated identification andalerting systems, and vehicle access control systems. As is mentionedabove, current automatic license plate recognition systems have captureefficiencies that are lower than desired due to, for example, low orinconsistent contrast of indicia as well as obscuring or distractingcontrast of artwork and/or indicia on the license plate.

The retroreflective articles of the present application may also be usedin signage. The term “signage” as used herein refers to an article thatconveys information, usually by means of alphanumeric characters,symbols, graphics, or other indicia. Specific signage examples include,but are not limited to, signage used for traffic control purposes,street signs, identification materials (e.g., licenses), and vehiclelicense plates. It would advantageous in some applications to use thearticles of the present application to employ the desirable property ofviewing machine-readable bar codes without changing the appearance of asignage under visible light. Such retroreflective articles would enablethe reading of signage specific information meant for generalconsumption while avoiding driver or sign reader distraction by and/orunwanted detection of “covert” markings, such as variable information ofa bar code. Such a development facilitates invisible marking of and/orsignaling by articles for security purposes, identification, andinventory control. For example, the covert markings could containsignage-specific information such as, for example, signage material lotnumber, installation date, reorder information, or product lifeexpectancy.

Objects and advantages of the present application are furtherillustrated by the following examples, but the particular materials andamounts thereof recited in the examples, as well as other conditions anddetails, should not be construed to unduly limit the invention, as thoseof skill in the art will recognize that other parameters, materials, andequipment may be used.

Example 1

The retroreflective article shown in FIGS. 1-5 was prepared: a roll ofwhite reflective sheeting (i.e., retroreflective substrate) 40 withpressure sensitive adhesive coated on the backside (commerciallyavailable under the designation “Preclear Reflective License PlateSheeting Series 4790” from 3M Company, St. Paul, Minn.) was provided.Alphanumeric characters 66 and background graphic 68 were printed on thefrontside of a 15 cm by 31 cm sample of the reflective sheeting using anUV inkjet printer (model “JF-1631”, commercially available from MimakiUSA, Suwanee, Ga.) and visibly-opaque, infrared-transparent, cyan,magenta and yellow (CMY) inks (commercially available under the tradedesignation “Mimaki UV Piezo Inkjet Ink Series F-200” from 3M Company).The background graphic 68 was a wooded lake scene with the words“Explore Minnesota.com” 62 printed in blue across the upper portion ofthe retroreflective sheeting, and the words “JAN” 63 and “10,000 Lakes”64 printed in blue across the lower portion of the retroreflectivesheeting. Alphanumeric characters “001 SPL” 66 were printed in black,using a combination of cyan, magenta and yellow inks, on the approximatecenter of the sheeting. The printed sheeting was adhered to an aluminumsubstrate 20. The frontside of the sheeting was then laminated with“Clear Protective Film 9097” film commercialized by 3M Company.

Embedded machine-readable information 80 (e.g., partial characters) wasprepared by selectively adhering pieces of visible andinfrared-scattering, pressure-sensitive adhesive tape “Scotch MagicTape” (commercially available from 3M Company) over and selectivelycovering portions of the “001 SPL” characters 66.

FIG. 2 is a digital photograph of the retroreflective article 10 ofExample 1 taken with a digital camera (model “PowerShot G12”,commercially available from Canon U.S.A., Lake Success, N.Y.) underambient conditions (e.g., diffuse light). In this condition, theembedded machine-readable information 80 was invisible to the viewerand/or camera. FIG. 3 is a digital photograph of the retroreflectivearticle 10 of Example 1 taken with the “PowerShot G12” camera undervisible retroreflective conditions, using the built-in flash. Undervisible retroreflective conditions, the embedded machine-readableinformation 80 was concealed by the alphanumeric characters 66. FIG. 4is a digital photograph of the retroreflective article 10 of Example 1taken with a CCD (charge-coupled device) camera (model “Lu165”,commercially available from Lumenera Corporation, Ottawa, Ontario)equipped with a 50 mm focal length lens (model “C-56-531” commerciallyavailable from Edmund Optics, Barrington, N.J.), and a filter thatblocks visible light and allows near infrared light to pass through(model “R-72” commercially available from Edmund Optics) underretroreflective infrared conditions (i.e., wherein infrared illuminationis nearly coincident with the optical axis of the camera lens). Infraredillumination was provided by an array of 48 diodes surrounding thecamera lens emitting at a nominal wavelength of 810 nm. Because the inksused to print the alphanumeric characters “001 SPL” 66 wereinfrared-transparent, only portions of the alphanumeric characterscovered by the infrared-scattering “Scotch Magic Tape” were visibleunder retroreflective infrared conditions. FIG. 5 is a digitalphotograph of the retroreflective article 10 of Example 1 taken with the“Lu165” camera under off-axis infrared illumination conditions, whereinfrared illumination was not coincident with the optical axis of thecamera lens. Under off-axis infrared conditions, neither thealphanumeric characters nor the embedded machine-readable informationwas visible.

Framing information in Example 1 was provided by the alphanumericcharacters 66 and obtained from FIG. 2. Alternatively, framinginformation could have been obtained from FIG. 3. Embeddedmachine-readable information 80 was obtained from FIG. 4. Thealphanumeric characters “001 SPL” 66 contained 6 characters with threebits in each character, for a total of 18 bits. Under infraredconditions, a binary optical code was observed, wherein dark bits (e.g.,infrared-opaque and/or scattering) represented a “1” and light (e.g.,infrared-transparent) bits represented a “0”. Variable information forExample 1, when the bits were read top to bottom and left to right, was:1 1 1 0 1 0 0 1 1 0 0 1 0 1 1 1 0 1.

Example 2

The retroreflective article 100 shown in FIGS. 6-9 was prepared: aprinted retroreflective sheeting was prepared as described in Example 1,except that (1) the alphanumeric characters “SAM 123” 166 were printedusing the CMY inks; and (2) embedded machine-readable information 180was prepared by printing partial alphanumeric characters using a visiblyopaque, infrared-absorbing (e.g., infrared opaque), black ink(commercially available under the trade designation “Mimaki UV PiezoInkjet Ink Series F-200” from 3M Company). “Clear Protective Film 9097”film was laminated over the printed sheeting to provide aretroreflective article 100 with embedded machine-readable information180 therein. The partial alphanumeric characters of the embeddedmachine-readable information overlapped perfectly with portions of the“SAM 123” characters 166.

FIG. 6 is a digital photograph of the retroreflective article 100 ofExample 2 taken with the “PowerShot G12” camera under ambientconditions. Under ambient conditions, the embedded machine-readableinformation 180 was invisible to the viewer. FIG. 7 is a digitalphotograph of the retroreflective article 100 of Example 2 taken withthe “PowerShot G12” camera under visible retroreflective conditionsusing the built-in flash. In this condition, the embeddedmachine-readable information 180 was concealed by the alphanumericcharacters “SAM 123” 166 and invisible to the detector. FIG. 8 is adigital photograph of the retroreflective article 100 of Example 2 takenwith the “Lu165” camera under retroreflective infrared conditions.Similarly to FIG. 4, only the embedded machine-readable information 180was visible under retroreflective infrared conditions. FIG. 9 is adigital photograph of the retroreflective article 100 of Example 2 takenwith the “Lu165” camera under off-axis infrared illumination conditions.Under off-axis infrared conditions, only the embedded machine-readableinformation 180 was visible.

Framing information in Example 2 was provided by the alphanumericcharacters 166 and obtained from FIG. 6. Alternatively, framinginformation could have been obtained from FIG. 7. Embeddedmachine-readable information 180 was obtained from FIG. 8.Alternatively, embedded machine-readable information 180 could have beenobtained from FIG. 9. The alphanumeric characters “SAM 123” contained 6characters each character having a different number of bits, as shown inTable 1, below, for a total of 91 bits. Under infrared conditions, theembedded machine-readable information comprised a binary optical code,having dark bits, which represented a “1”, and light bits, whichrepresented a “0”. Variable information for Example 2, when the bitswere read from left to right tracing the characters, was:

1011011010001001101111011010101101011011110011011010101000110100101101100100110110111001101

TABLE 1 Number Variable information visible Character of Bits underinfrared conditions “S” 14 10110110100010 “A” 17 01101111011010101 “M”21 101011011110011011010 “1” 12 101000110100 “2” 13 1011011001001 “3” 1410110111001101

Example 3

A printed article 200 having alphanumeric characters 266 andmachine-readable information 280 was provided as described in Example 2,except that (1) an optically inactive substrate (white vinyl sheeting(obtained under the trade designation “CONTROLTAC Graphic Film IJ180-10”available from 3M Company) was used, and (2) no clear film was laminatedover the printed sheeting.

FIG. 10 is a digital photograph of optically inactive article 200 takenunder ambient conditions using the “PowerShot G12” camera. Under ambientlight conditions, machine-readable information 280 was concealed by thealphanumeric characters “SAM 123” 266. FIG. 11 is a digital photographtaken under off-axis infrared illumination with the “Lu165” camera. Onlythe machine-readable information 280 was visible under this condition.

Example 4

A retroreflective article 300 was prepared as described in Example 1,except that (1) no alphanumeric characters were printed; (2) a 1 in(2.54 cm) tall and 6.5 in (16.5 cm) wide machine-readable informationpattern 380 depicted on FIG. 13 was printed on the retroreflectivesheeting using a thermal transfer printer (obtained under the tradedesignation “3M Digital License Plate Printer”, from 3M Company) and awhite ribbon (obtained under the trade designation “Thermal TransferRibbon TTR 1321”, from 3M Company); and (3) a 1 in (2.54 cm) tall and6.5 in (16.5 cm) wide human-readable red rectangle 371 was printed overthe machine-readable information using red ink (obtained under the tradedesignation “3M Process Color Series 990”, from 3M Company).

FIG. 12 is a digital photograph taken under visible ambient conditionsusing the “PowerShot G12” camera. Under this condition, machine-readableinformation 380 was concealed by the human-readable rectangle 371. FIG.13 is a digital photograph taken with the same camera in a darkened roomusing the built-in flash and saturating the background, providingvisible on-axis illumination. Under retroreflective visible conditions,machine-readable information 380 was visible and human-readablerectangle 371 was invisible.

Those having skill in the art will appreciate that many changes may bemade to the details of the above-described embodiments andimplementations without departing from the underlying principlesthereof. The scope of the present application should, therefore, bedetermined only by the following claims.

What is claimed is:
 1. A retroreflective article comprising: aretroreflective sheeting; and an image layer including human-readableinformation and machine-readable information; wherein at least one ofthe human-readable information or machine-readable information includesa visibly-opaque and infrared transparent ink; and wherein themachine-readable information is contained within the boundaries of thehuman-readable information.
 2. The retroreflective article of claim 1,wherein the retroreflective sheeting includes one of cube cornerelements or microspheres.
 3. The retroreflective article of claim 1,wherein the retroreflective sheeting further comprises a body layer anda structured layer, the body layer having a first side and a secondside.
 4. The retroreflective article of claim 3, wherein the image layeris disposed on the first side of the body layer.
 5. The retroreflectivearticle of claim 3, wherein the image layer is disposed on the secondside of the body layer.
 6. The retroreflective article of claim 1,wherein the retroreflective sheeting comprises one of a seal film or areflective coating.
 7. The retroreflective article of claim 1, whereinthe human-readable information includes the visibly-opaque andinfrared-transparent, and the machine-readable information includes aninfrared-opaque ink.
 8. The retroreflective article of claim 1, whereinthe machine-readable information includes a visibly-opaque andinfrared-opaque ink.
 9. The retroreflective article of claim 1, whereinthe human-readable information includes a visibly-opaque andinfrared-transparent ink.
 10. The retroreflective article of claim 1,wherein the human-readable information includes a visibly-transparentand infrared-opaque ink, and the machine-readable information includes avisibly-opaque and infrared-transparent ink.